Packaging is one of the final steps in the process of manufacturing semiconductor chips. In packaging, a fabricated semiconductor chip is mounted within a protective housing. At the present moment, the art of semiconductor chip technology has evolved far more rapidly than the integrally related technology of packaging the semiconductor chips. The packaging requirements of the newer, smaller, more powerful semiconductor chips are quickly progressing beyond the capabilities of traditional packaging technology and the conventional materials and designs presently utilized are fast becoming obsolete. The packaging demands of new semiconductors require configurations to accommodate increasing numbers of electrical interconnections, space constraints due to decreasing size, reduction in costs, improved reliability, and increasing heat transfer capabilities.
The need to adequately transfer heat out of increasingly smaller semiconductor packages has spawned significant interest in the development of new packaging materials and more thermally efficient configurations. Currently, semiconductor packaging utilizes the art of attaching an external heat sink to improve the heat transfer characteristics of many chip packages which, by themselves, do not adequately transfer heat away from the semiconductor chip. However, with the increasing heat concentration of new semiconductor chips the standard addition of an external heat sink is no longer sufficiently thermally efficient.
In packaging technology, the two most significant modes of heat transfer are heat conduction and heat convection. The removal of heat through a medium or between media is based on temperature differential, i.e. heat flowing from hot to cold, until an equilibrium is reached. Both convection and conduction heat transfer modes are paramount in evaluating the heat transfer characteristics of semiconductor packages and/or attached heatsink members.
The heat dissipating characteristics of semiconductor packages are measured by a network of heat transfer pathways through which heat must flow. The heat must be conducted from the chip through various thermal pathways to reach an outer surface of the package exposed to a cooling fluid, such as air. The fluid then convects away the heat.
Every packaging material has its own unique thermal characteristics. One of these characteristics is known as thermal conductivity. The thermal conductivity of a material determines the amount of heat that can be conducted through that material. Some materials (such as metals) have high thermal conductivity, and other materials (such as rubber or glass) have low thermal conductivity. Common semiconductor packaging materials, such as glass, glass ceramic composites, and plastics, which allow for ease of manufacturing, unfortunately also feature low thermal conductivities which hinder the dissipation of heat.
Depending on the construction of the semiconductor package, a package may have a multitude of various materials of various thermal conductivities through which heat must travel to reach the cooling fluid. This path may be tortuous when many materials with low thermal conductivities are encountered.
Once heat from the semiconductor chip has been conducted through the various materials of the package and reaches the surface of the package it must be convected away to a cooling fluid, such as air.
As noted previously, heat transfer from the external surfaces of semiconductor packages is normally handled by the attachment of specially configured heat sinks which, generally, are mounted over the areas of greatest heat generation. This allows the heat to be conducted from the surface of the package, which usually has a low thermal conductivity, into the heat sink, which preferably has a high thermal conductivity and a large surface area. The heat then is convected away to the ambient air mass. The shapes of the heat sinks are configured to allow for a large surface area which increases heat convection away from the package. In previous applications, where the heat generation of semiconductor chips was moderate, the additional heat transfer capability created by externally bonding the heat sink to the package was sufficient to transfer any heat generated by the chip.
However, as heat generation has increased in the newer semiconductors, the effectiveness of externally mounted heat sinks has decreased. The thermal pathways of the package and heat sink can no longer adequately transfer the heat generated and can subsequently cause malfunctions within the semiconductors.
A major heat transfer problem common to many semiconductor packages is the configuration of the package and the mounting location of the semiconductor chip. Advantageously locating the chip can eliminate several thermal boundaries and improve heat transfer. Because the chip is routinely attached to an internal substrate, the heat generated from the chip must pass through the attachment means to the substrate before it reaches the heat sink mounted on the outer surface of the package. The chip-substrate attachment bonding material adds thermal barriers to the package and limits the maximum heat transfer capability of the package.
Plastic packages introduced decades ago, currently are utilized to package the vast majority of integrated circuits in the semiconductor chip industry. It is estimated that plastic packaging accounts for about 95 percent of the world market in semiconductor chip packaging (or about 40 billion parts per year).
A typical plastic packaging operation involves the following sequence of steps:
1. In order to hold the chip in place, it first is bonded, generally using an epoxy, to a flat surface, commonly referred to as a "paddle", in the center of a lead frame. PA1 2. The chip, held in place on the paddle, then is wire bonded to the lead frame. PA1 3. The lead frame and chip assembly is set within the cavity of a transfer molding fixture and a plastic composition is transferred to fill the cavity, thus encasing the chip, epoxy, paddle, bond wires, and part of the lead frame. PA1 4. The encased assembly is removed from the transfer molding fixture, and the wire leads are trimmed, plated, and formed to specific desired shape. PA1 5. If required for heat dissipation, a heat sink is bonded to the encased assembly.
In standard plastic package assembly, the heat generated by the chip must pass through the epoxy, paddle, and through the plastic encasing the chip, before it reaches the exterior of the plastic. Such an arrangement results in very poor thermal performance, since the epoxy and encasing plastic are poor thermal conductors.